Method of vacuum evaporation

ABSTRACT

A METHOD OF VAPOR DEPOSITING A UNIFORM LAYER OF MATERIAL ON THE SURFACE OF CYLINDRICAL SUBSTRATE WHICH COMPRISES ROTATING SAID SUBSTRATE SIMULTANEOUSLY ABOUT ITS PERPENDICULAR AXIS AND LONGITUDINAL AXIS WHILE MAINTAINING THE SUBSTRATE AT A PLANE INCLINED TO THE HORIZONTAL, WITH A SMALL SURFACE SOURCE OF EVAPORANT LOCATED BELOW SAID ROTATING SUBSTRATE, WITH THE RATIO OF THE RADIUS OF THE SUBSTRATE TO THE SOURCE-TO-SUBSTRATE DISTANCE BEING MAINTAINED FROM ABOUT 0 TO 1.0, SAID EVAPORANT SOURCE EMITTING A VAPOR FLUX WHICH DEPOSITS AS A SUBSTANTIALLY UNIFORM LAYER UPON THE SURFACE OF SAID SUBSTRATE BY MAINTAINING A SLOTTED MASK BETWEEN THE SUBSTRATE AND EVAPORANT SOURCE. WHILE MAINTAINING SAID SUBSTRATE AND EVAPORANT SOURCE UNDER VACUUM CONDITIONS DURING THE ENTIRE EVAPORATION CYCLE.

1973 w. s. LITTLE, JR 3,752,691

I METHOD OF VACUUM EVAPORATION wFiled June 29, 1971 4 Sheds-Shut 1 INVENTOR WILLIAM S. LITTLE JR.

1973 w. s. LITTLE, JR 3,752,691

METHOD OF VACUUM EVAPORATION Filed June 29, 1971 4 Sheets-Shoot 2 :02 BEST UNIFORMITY AT p-mr 50- LOI pus

o.9s- FIG. 2

Filed June 29, 1971 W. S. LITTLE, JR

METHOD OF VACUUM EVAPORATION BEST UNIFORMITY AT 002 p H. T

4 Sheets-Shut (5 1973 w. s. LITTLE, JR 3,752,691

METHOD OF VACUUM EVAPORATION Filed June 29, 1971 4 Sheets-Shut 4 United States Patent O US. Cl. 117-38 4 Claims ABSTRACT OF THE DISCLOSURE A method of vapor depositing a uniform layer of material on the surface of cylindrical substrate which comprises rotating said substrate simultaneously about its perpendicular axis and longitudinal axis while maintaining the substrate at a plane inclined to the horizontal, with a small surface source of evaporant located below said rotating substrate, with the ratio of the radius of the substrate to the source-to-substrate distance being maintained from about to 1.0, said evaporant source emitting a vapor flux which deposits as a substantially uniform layer upon the surface of said substrate by maintaining a slotted mask between the substrate and evaporant source, while maintaining said substrate and evaporant source under vacuum conditions during the entire evaporation cycle.

BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for vacuum coating.

In many commercial applications of vacuum coating, the deposition of a film or coating must display a uniform thickness over relatively large surface areas. Although many approaches with respect to apparatus modifications may be considered in attempting to maintain uniformity of coating thickness, they all are limited to a certain extent by cost, practicality, and the size or constraints of the vacuum chamber.

In conventional vacuum apparatus used for thermal evaporation, an evaporation boat containing the material to be evaporated is placed in a particular location within the vacuum chamber. The substrates to be coated are usually placed in a stationary location, usually above the evaporation source. The source material is then heated by any suitable means such as induction or resistance heating or electron bombardment. Particles of the evap' orated material pass in straight lines from the source to the stationary substrate and deposit as a coating on the substrate positioned above the heated source.

The above method and apparatus, however, have some severe limitations and disadvantages when it is required to use a plurality of evaporation boats and/or to coat a plurality of substrates and/or to deposit an extremely uniform film on a sample that is nearly as large as the vacuum chamber. For example, when using more than one evaporation boat, all of the boats cannot be positioned in the same location to insure that a coating of a uniform thickness will be obtained. One method of avoiding this problem is to rotate a substrate or plurality of substrates about a centrally located evaporation source. This technique is described in US. Pat. No. 3,128,205. It can be seen, however, that the apparatus disclosed in US. Pat. No. 3,128,205 would probably require a relatively large vacuum chamber in view of the relatively complex arrangement which is required for rotating the substrates above the evaporation source.

In order to simplify conventional vacuum coating apparatus and to assure high thickness uniformity as well as simultaneous deposition on several samples up to 6 inches in diameter, K. H. Behrndt in the Transactions of 10th National Vacuum Symposium, 1963; MacMillan Co. (New York), p. 379 in an article entitled Thickness Uniformity onRotating Substrates has devised an im- 'ice proved method of vacuum coating. In brief, the substrates to be coated are positioned horizontally and are located in a circle concentric to a small area source. While the evaporated material is being deposited, the substrates to be coated are rotated about their axis. Behrndt illustrates mathematically which set of design parameters will yield the highest uniformity of coating thickness for varying shape and diameter of samples. Behrndt shows that uniform coating may be obtained even under conditions Where the substrate dimensions are: nearly as large as the vacuum chamber dimensions if (1) The substrate is rotated at a constant velocity about its normal axis, and

(2) The substrate is located horizontally and off to one side so as not to be directly above the evaporation source.

He derives mathematically the optimum location for best uniformity, based on the various physical dimensions of the system.

It has been discovered that an extremely high degree of film uniformity may be obtained by orienting the substrate in a non-horizontal position either directly above or eccentric to the evaporant source. It has been learned that not only a single optimum location for a substrate for the best coating uniformity exists, but rather, there exists a whole family of optimum plate orientations. Thus the operator has much more flexibility in choosing acceptable substrate positions for a given set of dimensions and constraints. In addition to improving uniformity of the coating thickness, it has been demonstrated that relatively large substrates can be uniformly coated in relatively small vacuum chambers, thereby increasing efliciency and reducing apparatus cost.

OBJECTS OF THE INVENTION It is therefore an object of this invention to provide an improved method of vacuum coating which overcomes the above noted disadvantages.

It is another object of this invention to provide a method of vacuum coating large cylindrical substrates within a relatively small vacuum chamber.

It is another object of this invention to provide a method of vacuum deposition which exhibits a high deposition efficiency.

It is a further object of this invention to provide a method of vacuum deposition which exhibits a high uniformity in coating thichkness.

SUMMARY OF THE INVENTION This invention is directed to a method for thermal evaporation under vacuum conditions using a small surface source such as a crucible, boat, or the like. Through the use of the method and apparatus of the instant invention, means are provided for obtaining extremely uniform vacuum evaporated films or coatings on a substrate which is placed at a precisely determined position within a vacuum evaporation chamber above a small surface source of evaporant. The invention includes rotating the substrate about its normal axis with a constant angular velocity at a predetermined angle of tilt or inclination from the horizontal position. By using this technique, the substrate to be coated comes closer to the source in the region of smaller particle flux and farther from the source in the region of larger particle flux, so that the average material deposited during a large number of revolutions is nearly constant over a large surface area. One example of the instant invention allows the coating of 4 inch diameter cylinder 9 inches long in a 18 x 30 inch vacuum bell jar and maintaining a thickness variation within about 0.2 percent. In the specification it should be understood that the descriptions relating to planar substrates are equally applicable to a rotating cylindrical substrate which is masked to allow only a narrow strip on the side toward the evaporation source to be exposed to the particle or vapor flux.

It is well known that the vapor flux radiating from an ideal surface source is inversely proportional to the square of the distance from the source. Further, in many practical cases it is found that the vapor flux is directly proportional to the cosine of the angle between the normal to the source surface and the direction of the vapor flow. This ideal type of source is used frequently in vacuum deposition analysis because surface sources can be closely approximated experimentally. They are also easily handled mathematically if the source dimensions are small compared to the distance to the substrate. The evaporant is placed in a boat whose aperture is large compared to the height of the walls above the level of liquid evaporant. In the article by Behrndt referred to above, an expression is derived for the vacuum deposited film thickness on a rotating spherical or planar substrate. This article treats only those cases in which the substrate axis and the perpendicular to the source surface are parallel to each other (both vertical). Therefore, for planar substrates those results apply only for those plates lying in a horizontal plane somewhere above the source.

In the instant invention Behrndts analysis is generalized to include all possible orientations of the axis of rotation, but treats only planar substrates. While Behrndts results of planar substrates indicate a single optimum location of the substrate for best coating uniformity, the more general problem gives rise to a whole family of optimum plate orientations. Within each family there is a trade off between the angle of inclination of the substrate and the angular displacement of the plates center from the vertical axis through the source. It turns out, in one case for example, that the plate axis of rotation should be inclined about 46 with respect to the vertical when the plate is centered directly above the evaporation source. This optimum inclination angle changes to about 11 when the center of the plate is about 28 off to the side of the normal to the source, and changes further to about when the plates center is 36 off to the side of the normal to the source. Each geometry gives rise to highly uniform deposition on a 12" diameter plate 24" away from the source. The operator may select any one of these plate locations, or any other intermediate plate location, which is consistent with his vacuum chamber dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and various features of the above invention will become apparent upon consideration of the following detailed description of the invention especially when taken in conjunction with the following drawings wherein:

FIG. 1 is a schematic view of one embodiment of the instant invention which illustrates by rectilinear coordinates the relative position of a planar substrate and an evaporation source.

FIG. 2 represents a family of surface profiles representing the variation in film thickness as a function of points on the substrate at various distances from the center. The entire family applies to a single location of the plates center. Each curve represents a different angle of inclination of the plate.

FIG. 3 represents a family of surface profiles of varying film thickness for a second position of a substrate and evaporation source.

FIG. 4 is a schematic sectional view of one embodiment of an apparatus suitable for rotating and varying the angle of inclination of a substrate from the horizontal.

FIG. 5 is a schematic view of a slotted mask which is used to control the vapor flux during evaporation.

FIG. 6 is a schematic view of a second embodiment of apparatus suitable for rotating and inclining a substrate.

4 DETAILED DESCRIPTION OF 'THE INVENTION A plot of the expected film thickness is obtained by calculating the thickness T at various points S inches from the substrates center P using Equation 1, where T is the film thickness at P, and R is the distance from P to the evaporant source.

cos B een steer" (Equation 1) The parameters X, Y and Z are the rectilinear coordinates of P as measured from the evaporant source in the coordinate system illustrated in FIG. 1, and

The calculations are repeated for various angles of inclination 5 of the substrate holder from the horizontal position. In this way, a complete set of families of film thickness profiles is plotted for a geometrical arrangement of fixed X, Y and Z of the coating apparatus. One may then select what appears to be the best profile for a particular application and then adjust the inclination angle ,8 of the substrate to the corresponding value prior to evaporation.

For example, if the vacuum equipment makes it necessary to place the substrate center P directly above the substrate source, then Equation 1 is reduced to Equation 2.

lt ttfl df t (Equation 2) FIG. 2 illustrates the resulting family of surface profiles described by Equation 2. In the example, a substrate one foot in diameter is centered 2 feet directly above the source giving rise to a range of interest of S/R of from O to 0.25. The curves show that for this range of S/R, highly uniform films will result when ,8 is about 46. The thickness variation under these conditions will be less than 0.2 percent over the entire substrate.

In the second case, if X=0.7 foot, Y=0.7 foot, and U:1.73 feet (making R=2 feet), then Equation 1 yields the profile shown in attached FIG. 3. In this case, the one foot diameter substrate inclined at a 3 of -l1 would be uniformly coated to within 0.2 percent.

The table below shows those angles 18 which exhibit optimum uniformity for various values of the relative coordinates X/R and Y/R. The data in this table was obtained by examining multiple plots of Equation 1 for each (X/R, Y/R) pair and selecting the most uniform coating or film profile. The relation is used in conjunction with Equation 1. T T 0 is plotted as a function of S/R for various 5. The data in the table apply to the case where the substrate radius is 0.25 of the source-to-substrate distance R, but they are nearly the same as those data which would be obtained for plates whose radii lie anywhere in the range of 0.2 to 0.4 relative to the distance R.

It is apparent that the method described herein for determining optimum substrate orientations is applicable to a wider range of substrate sizes than is covered by the particular data in the table. The method can be successfully applied to planar substrates of radii ranging from 0.0 to 1.0 times R, and higher.

The range 0.1 to 0.8 is considered a preferred range for the relative substrate radius because, below 0.1, the substrate is far enough removed from the source of evaporant that a highly uniform coating can be obtained by simply placing the substrate in a horizontal position directly above the source. Above 0.8, the substrate is likely to extend so far to the side or so far downward toward the plane of the source that it would extend into regions where the flux strength deviates from a true cosine law.

Apparatus suitable for rotating a cylindrical substrate about its horizontal axis end and inclining or tilting the cylinder to the appropriate angle of inclination from the horizontal axis is set forth in FIG. 4. This apparatus comprises two upright support members and 11, containing a rotary power means 12a. The rotary power means 12a contains an outer housing 12, which contains therein the shaft member 13 containing bevel gear 14. Bevel gear 15 which is meshed with bevel gear 14, is connected to shaft 16 which is connected to spur gear 17 which is meshed with spur gear 18. Gear 18 is connected to bevel gear 19 which is meshed with bevel gear 20 which is connected to a drive shaft 21 which is connected to drive motor which is not shown. Shaft member 13 is further connected to cylinder holder 22 by a flange 23. Cylinder holder 22 holds a cylinder 24, adapted for being rotated about its longitudinal axis through pin 25 which is contained within bushing 26, and slotted pin 27 which is connected to bushing member 28, which is further connected to a power source (not shown) for rotating cylinder 24 about its longitudinal axis. The angle of inclination is formed by moving power means 12a and cylinder holder 22 which rotate about pins 29 and 31 contained within bushings and 32. This angle of inclination is more clearly shown in FIG. 1 of the drawings. A slotted mask 33, containing a narrow slot 34 (better illustrated by FIG. 5) is attached to the bottom of cylinder holder 22. This mask allows only a narrow strip on the surface of the drum, which is toward the evaporation source, to be exposed to the evaporating particle flux.

In operation, the apparatus in FIG. 5 functions to rotate cylinder 24 about its horizontal axis by the rotation of drive shaft 21 actuating gear arrangement 19 and 20. This causes gear arrangement 17 and 18 to rotate causing shaft 16 to rotate gear arrangement 14 and 15, which results in shaft 13 rotating cylinder holder 22 about the horizontal axis of cylinder 24. Cylinder 24 also is simultaneously rotated about its longitudinal axis.

An alternative means for rotating and inclining the substrate is illustrated by FIG. 6 in which a series of pulleys 40, 41 and 42 are driven by a chain member 43.

The instant invention is directed to thermal vacuum evaporation in which a small surface source such as a boat or crucible is heated by any suitable source to emit a deposition flux which is coated onto a substrate within a vacuum chamber. The present invention has utility for coating any suitable material onto any suitable substrate: for example, metallic oxide, inorganic or organic materials, may be coated in thin films onto any suitable substrate materials such as glass, plastic, ceramic, metal, paper, etc. The vacuum conditions and boat temperature naturally vary according to the vapor pressure of the deposition material, and type of substrate but generally lies in the range of about 10- to 10 torr are generally satisfactory.

.A preferred application of the instant invention includes the vacuum deposition of a thin film of vitreous selenium and vitreous selenium alloys on a conductive substrate for use as a xerographic plate or drum.

Although specific components and proportions have been stated in the above description of the preferred embodiments of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials and changes may be utilized which synergize, enhance or otherwise modify the method of the instant invention.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be within the scope of this invention.

What is claimed is:

1. A method of vapor depositing a uniform layer of material on the surface of cylindrical substrate which comprises rotating said substrate simultaneously about its perpendicular axis and longitudinal axis while maintaining the substrate at a plane inclined to the horizontal, with a small surface source of evaporant located below said rotating substrate, with the ratio of the radius of the substrate to the source-to-substrate disance being maintained from about 0 to 1.0, said evaporant source emitting a vapor flux which deposits as a substantially uniform layer upon the surface of said substrate by maintaining a slotted mask between the substrate and evaporant source, while maintaining said substrate and evaporant source under vacuum conditions during the entire evaporation cycle.

2. The method of claim 1 in which the ratio of the radius of the substrate to the source-to-substrate distance is from about 0.1 to 0.8.

3. The method of claim 1 in which the inclined substrate is located substantially directly above the evaporant source.

4. The method of claim 3 in which the substrate is inclined at an angle of about 46 degrees.

References Cited UNITED STATES PATENTS 2,456,241 12/1948 Axler et al 1l7-l07.1 3,128,205 4/1964 Illsley 1l7--l07.1 3,628,994 12/1971 Blecherman 117-107.1 3,297,475 1/1967 Flacche 117-106 R 3,558,351 l/19'7l Foster ll7-l06 R 3,594,227 7/ 19'71 Oswald 117-106 A 3,632,406 1/ 1972 Clough et a1. l17l07.1 3,636,492 l/1972 Yamashita et a1. 1l7l05.4

WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R. 

